The field of biomedical engineering currently embraces a variety of endoprosthetic solutions: in a first such solution, disclosed in EP 461 791, the resulting stent takes the form of a small metallic tubular element of which the cylindrical surface is pierced by micro slots or cuts; when installed in a blood vessel, the tubular element is enlarged and deformed, permanently, in such a way that the slots are expanded to form diamond apertures and the tube is effectively transformed into a mesh duct.
In a second solution, disclosed in EP 282 175, the tubular element of the stent consists in a single metal wire formed initially into a flat serpentine appearing as a plurality of elongated loops with rectilinear members and interconnecting bends, and with no break in continuity; the flat element is compressed radially over the balloon catheter and inserted into the coronary vessel, then spread to the required dimensions by expanding the balloon, exploiting a "mechanical memory" incorporated into the selected wire material.
In other solutions, as disclosed in EP 312 852 and EP 378 151, use is made of single spiral wound metal wires, similarly implanted by means of a balloon catheter, of which the configuration and the diametral and axial dimensions can be varied likewise thanks to the "memory" of the particular metal selected.
"Memory" in this context signifies the facility whereby a material can be manipulated when exposed to particular mechanical, thermal or environmental conditions, to the end of bringing about a change in configuration; for example, in the case where a wire or mesh is made from an alloy of titanium and nickel, or from stainless steel or music wire, or from an annealed metal, change is induced by virtue of a mechanical memory, and in the case of certain materials such as the aforementioned titanium and nickel alloy, by virtue of a thermal memory.
It is known and scientifically proven that the successful manufacture of coronary endoprostheses guaranteeing both safety over time and operational efficiency is dependent on key requirements linked to mechanical and physiological factors: firstly, there is the need to ensure a sound resistance to radial deformation, given that the artery functions as a means of conveying blood under pressure and in consequence is subjected to a pressor action that the stent must be able to withstand; and secondly, there must be no effects generated that may upset the normal functioning of the artery.
In view of this second requirement, it is in fact inadvisable to fashion a stent exhibiting extensive surfaces fragmented by relative slots or openings; over time, these can create interstices favouring the build-up of atherosclerotic plaque, of which the physical impact is not inconsiderable. On the other hand, compact stents fashioned with extensive contact surfaces and more consistent thicknesses can lead to the risk of thrombosis.
It is therefore essential in manufacture to ensure a favourable balance between the surface area of the stent and the gauge of the metal utilized.
Accordingly, the object of the invention is to overcome the aforementioned drawbacks.